Most animals and plants reproduce sexually this means that DNA is passed down

Table of Contents Show

The two methods of reproduction
Examples of asexual reproduction
Sexual reproduction
Further development after fertilisation
Summary of differences
Differences between male and female gametes
Do most animals and plants reproduce sexually?
How do most plants reproduce sexually?
What happens when plants reproduce sexually?
Why do most animals reproduce sexually?

The two methods of reproduction

This unit has recently been expanded - 2 notes at the end

Asexual reproduction means reproducing without the interaction of two sexes or genders, whereas sexual reproduction involves the fusion of two special cells called gametes, one from a male source and one from a female source.

Before a cell divides, its nucleus divides. Each chromosome is copied, and each nucleus receives the same genetic material: genes, made of DNA.

As each cell divides into two, the resulting "daughter" cells are therefore exact copies of one another.

This process is responsible for the increase in number of cells which occurs during normal growth and development, and when tissues are replaced following injury.

Normal cell division is also the basis for asexual reproduction. Only one type of cell is involved, with no input from another individual. Because no new genetic material is introduced, there is no variation in the resulting offspring.

Since the offspring from this process contain the same genetic material as one another (and the same as the original single parent), they can be described as a clone.

Examples of asexual reproduction

Asexual reproduction in plants

There are many examples of asexual reproduction in plants, e.g. the spider plant Chlorophytum which produces plantlets on stolons branching from buds in the parent plant.

Bryophyllum (Kalanchoe) has plantlets (with dangling roots) along the edges of its leaves. These easily become detached and grow.
These resulting plantlets and plants are genetically identical and will grow to look alike, provided that they are raised in the same environment.
What features of the plant's environment would be need to be standardised (for them to look the same)?
>same illumination (amount of light)
>similar temperature
>watered similarly
>same sort of soil (minerals etc)

Many plants used for food can be propagated, i.e increased in number, by the method of asexual reproduction.

Do not confuse asexual reproduction with (sexual) reproduction in flowering plants, which often combine both male and female parts in the same flower.

Fruits and seeds are produced as a result of sexual reproduction.

Some parts of plants become enlarged as a result of normal cell division and this is called vegetative growth. This is often linked with surviving adverse weather conditions and keeping food reserves for the plant in order to grow again the next season. These plants are called vegetables, and Man often uses these reserves for himself.

Each of the examples of food plants below uses asexual and sexual reproduction in different ways

Asexual reproduction in bacteria

Asexual reproduction in animals

Asexual reproduction is much less common in animals, but it is often seen in simpler animals e.g. Hydra.


Hydra with 2 buds (one not yet showing tentacles)	Aphid (greenfly) giving birth	Identical twins are produced by a form of asexual reproduction when the ball of cells making up the embryo breaks into two, and each implants in the uterus and grows independently (after the normal sexual form of reproduction, obviously!)

In animal lifecycles, asexual reproduction sometimes alternates with sexual reproduction. See weblink below.

Sexual reproduction

Both male and female sex cells (sperms and eggs in animals, pollen and ovules in plants) are produced by a special cell division process which halves the number of chromosomes in each resulting cell. The chromosome separation process ensures that each sex cell has a unique combination of genes in its nucleus.

Fertilisation is also a random process and so when the nuclei fuse the resulting fertilised egg (zygote) has an individual genetic makeup.

In contrast to asexual reproduction, sexual reproduction introduces variation into offspring. This is an essential feature in order for evolution to take place.

Further development after fertilisation

This zygote then divides again and again using the normal process of cell division, producing cells containing genes which are exact copies of the original. So each cell of the embryo, and the adult organism into which it develops, contains cells which are genetically identical. This is fortunate because the body's immune system will target any "foreign" cells (normally invading microbes) which differ from the others.

However as each organ develops, the cells within it (collectively known as tissues) become specialised for their particular tasks, e.g. muscle cells, nerve cells, red and white blood cells, and they "read" and use only part of their genetic information to do this.

As they have differentiated into these different cell types, it appears that they have lost the ability to divide again into other cell types. A few unspecialised cells (e.g. stem cells) retain the ability to do this.

Summary of differences

	Asexual reproduction	Sexual reproduction
Number of parents	1 (either male or female)	Usually 2 (male and female) - see note 1 below
Makeup of offspring	genetically identical (to parent and other offspring)	genetically different
Cell division process	normal cell division following nuclear division (by mitosis)	special cell division following nuclear division (by meiosis) producing sex cells (gametes) - see note 2 below: after fertilisation subsequent divisions: normal
Advantages	quick - good for bulking up of numbers to colonise new areas	produces variation - the basis of evolution
Disadvantages	disease may affect all	slower - needs special processes to bring together gametes and protect zygote, embryo etc during development
Life cycle	useful when conditions ideal for growth	may be synchronised with (end of ?) growing season

Note 1

Some organisms are hermaphrodite (bisexual) - i.e. have both male and female sexual organs. This is found in many forms of invertebrates, e.g. molluscs, earthworms.

It is not so common amongst vertebrates, although some fish change from one sex to another.

Most flowering plants are hermaphrodite. In their stamens they produce pollen (spores acting as male sex cells) as well as ovules (female sex cells) produced in their ovaries.

Lily flower- showing both male and female parts

Hermaphroditism enables all individuals to be producing offspring, and encounters between pairs of organisms to be productive.
There are often mechanisms to prevent hermaphrodite organisms fertilising themselves, such as different timings of the various processes, and chemical incompatibility mechanisms.

When self-fertilisation occurs the result is not the same as asexual reproduction - the offspring being genetically identical to the parent. When sex cells are produced, only half the parent's genetic material is used and then combined with an equal amount of genetic material from the other sex cell. There are elements of randomness in the production of gametes and in the fertilisation process, so resulting offspring are at least partly genetically different from the parent, and one another. There would be always be variation in a population of sexually reproducing organisms so that some organisms are more suited to their environment than others and survival of the fittest will underpin the process of evolution.

[The term hermaphrodite may also be used in human terms to describe rare instances of uncertainty over a person's sex, perhaps resulting from a developmental disorder, but the term intersex is preferred.]

Note 2

Differences between male and female gametes

Male gametes

Female gametess

smaller
produced in large numbers
able to move - either of their own accord e.g. swimming sperm cells
or by other means e.g. pollen carried by the wind or pollinating insects
consequently more likely to suffer wastage, but this could be another example of survival of the fittest

Some (smaller, simpler) plants produce swimming sperm cells.
These are found in wetter places.

larger - with food reserves inside the cell
therefore fewer in number
not so inclined to move - the male gamete moves to them, not vice versa
usually more protected - and this protection continues to be given to the developing embryo - competition again?

However there are cases of simple organisms where two very similar sized cells fuse, then after nuclear fusion meiosis occurs - and there is the suggestion that sex itself can be seen as the result of evolution.

Web links

Do most animals and plants reproduce sexually?

Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants.

How do most plants reproduce sexually?

Flowering plants reproduce sexually through a process called pollination. The flowers contain male sex organs called stamens and female sex organs called pistils. The anther is the part of the stamen that contains pollen. This pollen needs to be moved to a part of the pistil called the stigma .

What happens when plants reproduce sexually?

Once pollen gets transferred to the stigma the male gametes from pollen grains release and fuse with the egg in the ovule to form a zygote. This process of fusion of gametes is called fertilization. The zygote thus formed, divides and develops into an embryo, and later into a seed. The ovary develops into a fruit.

Why do most animals reproduce sexually?

The majority of animal species reproduce sexually. Sexual reproduction is generally considered to be advantageous because it results in genetically variable progeny due to segregation and recombination events (Williams, 1975; Maynard Smith, 1978; Bell, 1982).